Alteration scenario for bentonite under hyperalkaline conditions with regard to mineral paragenetic uncertainty

強アルカリ条件下におけるベントナイトの変質シナリオ

小田 治恵  ; 佐々木 良一; Arthur, R. C,*; Savage, D.*; 本田 明

Oda, Chie; Sasaki, Ryoichi; Arthur, R. C,*; Savage, D.*; Honda, Akira

TRU廃棄物処分では大量にセメント系材料を用いることから、緩衝材とセメント反応水との反応により緩衝材の化学的・鉱物学的変化が生じ、その結果として緩衝材の物質移動特性が変化する可能性がある。このような緩衝材の化学的・鉱物学的変遷過程には不確実性があり、変遷経路を一つに定めて化学的・鉱物学的変化や特性変化を議論することは難しい。しかしながら、生起し得る複数の変遷経路を緩衝材の鉱物学的長期変遷シナリオとして抽出し、この複数のシナリオに基づいた物質移動・化学反応解析を実施することによって、緩衝材の鉱物学的変化と特性変化の幅を評価することが可能である。そこで、既往の知見に基づき、セメント反応水の影響を受けた緩衝材の鉱物学的長期変遷シナリオを作成した。

Bentonite materials are expected to be used as the main hydraulic barrier and cementitious materials are expected to be used for structure support in low level radioactive waste disposal systems. Interaction between bentonite and hyperalkaline fluids arising from cementitious materials has been recognized as a concern due to the close proximity of these materials in repository designs. The interaction could cause the alteration of mineralogy and associated hydraulic property of bentonite and have a deleterious influence on the function of bentonite as a hydraulic barrier, and may lead to adverse impact on the long-term safety of the repository. Many researches on such interaction have been performed to establish the bentonite alteration scheme, however, uncertainty in our understanding of the precise alteration scheme, especially mineral paragenetic sequences during the alteration, still remains. Therefore, it is important to take such uncertainty into account in estimating how the mineralogy and the hydraulic property of bentonite evolves subject to alteration by hyperalkaline fluids. In this work, multiple scenarios for mineralogical alteration of bentonite was investigated to limit the mineral paragenetic uncertainty and to identify appropriate assumption for numerical simulation of chemical and mineralogical evolution of bentonite under hyperalkaline conditions over the long-term.